The White House says it has "no evidence" to support Iran's claim it has captured a US drone that was flying in the country's airspace and gathering intelligence over Gulf waters.
The Iranian Fars news agency reported that the ScanEagle drone was captured "in the last few days" by the naval unit of the Islamic Revolutionary Guard Corps.
Ali Fadavi, the Revolutionary Guards' naval commander, was quoted by state TV as saying the unmanned aircraft was now in Iran's possession but declined to say how or where it was seized.
He said the drone "was conducting a reconnaissance flight and gathering data over the Persian Gulf" and "was captured by the Guard's navy air defence unit as soon as it entered Iranian airspace".
But White House spokesman Jay Carney, when asked about the downed drone, told a reporter: "We have no evidence that the Iranian claims you cite are true."
A US Navy spokesman earlier said no US drones were missing in the Middle East despite Iran's claims.
"The US Navy has fully accounted for all unmanned air vehicles (UAV) operating in the Middle East region. Our operations in the Gulf are confined to internationally recognised water and air space," Commander Jason Salata, from the US Navy's 5th Fleet, said.
"We have no record that we have lost any ScanEagles recently."
He added that US ScanEagles have been lost into the sea in the past.
However on Tuesday the US Navy said it was trialling a new type of UAV in the Bahrain-based 5th Fleet's area of responsibility.
Called the Pointer Upgraded Mission Ability - All Environment (Puma AE), it said the drone was designed for use by special force personnel.
It added the drone could be retrieved from water, with the ability to float for several hours before recovery.
"This gives us time to get to the craft if it lands far away," Master-at-Arms 2nd Class Daniel Meehan said.
"If the Puma AE is recovered by the wrong hands, they can only recover GPS information."
Last September, the US Navy's Riverine Squadron 2 tested the Puma AE drone onboard a British ship in the region.
The drone was tested on the Royal Fleet Auxillary (RFA) Cardigan Bay, while mine hunter HMS Shoreham patrolled nearby.
The Ministry of Defence told Sky News that UK forces did not operate any drones outside of its Afghanistan area of responsibility.
Meanwhile, if the drone loss is confirmed it will likely to escalate tensions between the US and Iran.
The two countries are locked in a dispute over Tehran's nuclear programme, which Washington and its allies believe is aimed at developing an atomic bomb.
Iran says its programme is peaceful, but the country has come under tough international sanctions.
Last month, Iran said a US drone had violated its airspace. The Pentagon said the unmanned aircraft had come under fire at least twice but was not hit, and insisted the Predator drone was over international waters.
In 2011 Iran showcased a captured stealth drone - allegedly operated by the CIA - after it entered Iranian airspace from its eastern borders with Afghanistan and Pakistan.
Tehran later said it jammed radio controls to the aircraft and retrieved data from the RQ-170 Sentinel, a top-secret drone made by Lockheed-Martin - manufacturer of manned U-2 and SR-71 spy planes.
:: The Boeing-manufactured ScanEagle, according to its website, is 4ft long with 10ft wingspan and can be launched from naval vessels.